Reciprocating pump

ABSTRACT

Reciprocating pump includes a piston contained in a cylinder which is continuously vented at one end, and the other end is alternately subjected to pressure and vented through a simplified valve and porting arrangement. During the pressure phase, the piston is moved in the direction of the continuously vented end, whereas return movement of the piston is caused by an external applied force upon venting the pressure. The valving is located in the piston for maximum simplicity and most direct porting, and a small diameter fixed porting tube may be used to supply pressure to the valving in the piston through porting. The valving and porting relationships between elements assure against pump stalling.

United States Patent 1 3,695,785 Vanderlaan 1 Oct. 3, 1972 [54]RECIPROCATING PUMP Primary Examiner-Robert M. Walker [72] lnvemor' ggggz z zzt g z t Attomey-Stephen M. Mihaly Mich. 49007 ABSTRACT [22]Filed: Nov. 23, 1970 Appl. No.: 91,653

Reciprocating pump includes a piston contained in a cylinder which iscontinuously vented at one end, and the other end is alternatelysubjected to pressure and vented through a simplified valve and portingarrangement. During the pressure phase, the piston is moved in thedirection of the continuously vented end, whereas return movement of thepiston is caused by an external applied force upon venting the pressure.The valving is located in the piston for maximum simplicity and mostdirect porting, and a small diameter fixed porting tube may be used tosupply pressure to the valving in the piston through porting. Thevalving and porting relationships between elements assure against pumpstalling.

//////////// //7//////;y///// f'/ A INVENTOR. ROBERT DALE VA/VDERLAA/VATTORNEYS PATENTEDnm m2 sum 1 or 5 PATENTEUnms mm 3.695. 785

smsnunrs INVENTOR. ROBERT DALE VANDERLAA/V ATTORNEYS RECIPROCATING PUMPBACKGROUND OF THE INVENTION This invention generally relates to areciprocating pump, preferably driven by a regulated gas'supply todevelop high fluid pressure at the outlet.

'Previous known pumps and motors of this same general type were usuallyprovided with a differential area piston having a smaller effective areaat one end which was continuously subjected to inletpressureand a largereffective area at the other end which was alternately subjected to inletpressure and-exhaust, causing the piston to reciprocate. To obtain suchalternate pressure and exhaust at the larger area end of the pistonusually required the use of relatively complicated valve and portingarrangements which not only made the pumps costly to manufacture butthey were also usually inefficient and slow acting. Moreover, they weresusceptible to stalling under certain operating conditions, particularlyduring slow startup or when the valve was located in an intermediateposition in which the pressure across the valve was balanced, preventingmovement.

SUMMARY OF THE INVENTION A principal object'of this invention is toprovide a reciprocating pump with an improved and simplified valve andporting arrangement for controllingpiston movements. One end of thepiston is cylinder continuously vented, and the other end is alternatelyvented and pressurized by valving which is desirably located in thepiston for maximum simplicity and most direct porting. During thepressure phase the piston is caused to move in one direction, and whenthe pressure is vented the piston is moved in the reverse direction byan external applied force.

In one form of the invention, a small diameter fixed porting tubeextending into the center of the piston supplies pressure to the valvewhile greatly reducing seal friction. Resilient seals are also desirablyused on all sealing lands to reduce cost and improve-efficiency bysubstantially eliminating fluid leakage betweenmoving parts. The portingbetween the porting tube and valve also prevents pumpstall-out-regardless of the position of parts.

To the accomplishment of the foregoing and related ends the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawings setting forth in detail certain illustrat iveembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the invention may beemployed.

BRIEF DESCRIPTION OF THE DRAWINGS In the annexed drawings:

FIG. I is a schematic view showing a typical hydraulic systemincorporating a reciprocating pump constructed in accordance with thepresent invention;

FIGS. 2 through are fragmentary longitudinal sections through apreferred form of pump constructed in accordance with this inventionshowing the various positions of the pump piston and porting and valvingmechanism during a complete pump cycle;

FIG. 6 is a fragmentary longitudinal section through the pump of theFIGS. 2-5-embodiment-Showing the valve plunger in an intermediateposition;

FIGS. 7 and 8 are fragmentary longitudinal sections throughmodifiedforms of pumps constructed in accordance with the present invention; and

FIGS. 9 through 11 are fragmentary longitudinal sec tions through yetanother form of pump in accordance with this invention, showing thevarious positions ofthe piston and porting and valving mechanism'duringa pump cycle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in detail to thedrawings and first especially to FIG. 1, there is schematically shown ahydraulic system 1 including a pump 2 and reservoir 3 containinghydraulic fluid which is directed to the pump 2 through a fluid line 5for discharge from the pump outlet 6 under high pressure. Check valves 7and 8 may be provided within the fluid lines associated with thereservoir and pump outlets to prevent reverse flow of the fluid. Thepump 2 requires a constant low pressure gas supply connected to the pumpinlet 9 for operation and develops a high hydraulic'pressure which is afunction of the relative areas of the piston acted upon by the inlet gaspressure and piston rod acting on the hydraulic fluid. A vent 10 isprovided for venting one or both sides of the piston to cause the sameto reciprocate, as described hereafter.

One form of pump 2 constructed in accordance with thisinvention is shownin detail in FIGS. 2 through 6, and includes a'housing 11 containinga'cylinder 12 in which is axially slidably received a piston 13. Thepiston rod 14 extends into a pumping chamber 15 to which hydraulic fluidis supplied from the reservoir 3 throughthe fluid line 5. An externalseal 16 on'the piston rod 14 prevents leakage of such hydraulic fluidaround the piston 'rodinto the rod end 17 of the cylinder 12 which iscontinuously vented through the vent opening 10.

Reciprocation of the piston 13 is caused by alternately pressurizing andventing the head end 18 of the cylinder 12, in a manner to be morefullydescribed hereinafter. With the head end 18 of the cylinder 12vented, the piston 13 is caused to move to the left by an externalapplied force acting on the outer end of the piston rod 14, which may beapplied by placing the reservoir fluid under pressure or by disposing anexternalspring 19 between the piston rod and outer end of the pumpingchamber 15 as shown. Movement of the piston 13 in the reverse direction(i.e., toward the right) is caused by supplying the inlet gas pressureto the head end 18 of the cylinder 12 which produces a force acting onthe larger area piston 13 substantially greater than the relatively lowopposing external applied force acting on the smaller area of the rod14. In one form of the present invention, a 500 psi gas supply was usedto develop 3,000 psi hydraulic pressure during the pressure stroke ofthe piston rod, and a hydraulic reservoir pressure of psi was used toreturn the piston when the pressure acting on the head end of the pistonwas dumped.

For alternately venting and pressurizing the head end 18 of the cylinder12, a valve plunger 20 is provided,

desirably contained within a bore 21 in the piston 13 for maximumsimplicity, more direct porting, and minimum cost and weight. Supplypressure is fed to the valve bore 21 through a relatively small diameterporting tube 22 extending from the head end of the cylinder into acentral longitudinal bore 23 in the piston 13 and piston rod 14. Theporting tube 22 is fixed and has a longitudinal passage 24 thereincommunicating with the gas supply inlet 9 to the pump housing 1 1. Apair of longitudinally spaced external grooves 25 and 26 in the outersurface of the porting tube 22 contain annular seals 27 and 28 whichremain in sealed engagement with the central bore 23 of the piston 13during reciprocation thereof. Between the annular seals 27 and 28 theporting tube 22 is externally grooved at 29 for a substantial length andthere is a transverse passage 30 in the porting tube for supplyingpressure from the longitudinal passage 24 to the external groove 29. Aport 31 in the piston 13 provides communication between the groove 29and valve bore 21 for supplying pressure to the valve bore 21 for allportions of the piston 13 within the cylinder 12.

The valve plunger 20 is stepped as shown to provide a differentialpressure area 32 intermediate its length which is acted on by the supplypressure enteringthe valve bore 21 through the port 31, urging the valveplunger 20 toward the right. The left end of the valve bore 21 is alwaysvented through a passage 33 in the piston 13 communicating with the rodend 17 of the cylinder 12 which is continuously vented through the ventopening as aforesaid. The right end of the valve bore 21, on the otherhand, is pressurized or vented depending on the relative position of thevalve plunger with respect to two longitudinally spaced trigger ports 34and 35 extending between the valve bore 21 and piston bore 23 containingthe porting tube 22. When the right end of the valve bore 21 is vented,the valve plunger 20 is caused to move to the right by the supplypressure acting on the differential area 32, whereas when the right endof the valve bore 21 is pressurized, the valve plunger 20 is caused tomove to the left because of the larger area at the right end of thevalve plunger 20 acted upon by supply pressure.

With the valve plunger 20 in the extreme left portion shown in FIG. 2,supply pressure entering-the valve bore 21 is ported to head end 18 ofthe cylinder 12 through an external groove 36 in the valve plunger 20and additional ports 37 leading to the head end of the piston, wherebythe piston is caused to move to the right against the bias of theexternal force acting on the outer end of the piston rod 14. As thepiston 13 ap-' proaches its extreme right position as further shown inFIG. 2, the first trigger port 34 passes over the porting tube seal 27and vents the right end of the valve bore 21 through the right end ofthe piston tube bore 23, vent passages 38, and rod end of the cylinder12. This allows 'the supply pressure acting on the differential area 32of the valve plunger 20 to snap the valve plunger to the extreme rightposition shown in FIG. 3 for venting the cylinder head pressure throughthe small port hole pattern 37 and vent passage 33 in the piston 13. Thesecond trigger port 35 was opened to the vented end of the porting tubebore 23 prior to the first trigger port 34 during movement of the piston13 to the right, but the second trigger port 35 was precluded fromventing the right end of the valve bore 21 by the valve plunger 20itself. When the valve plunger 20 reaches its extreme right positionshown in FIG. 3, the right end of the valve bore 21 is continued to bevented through passages 39 and 40 in the valve plunger which communicatethe right end of the valve bore with the second trigger port 35.

Venting of the head end 18 of the cylinder 12 soon causes the pressureto drop below a level at which the external applied force of the lowhydraulic return pressure or return spring 19 is sufficient to returnthe piston 13 to the left as shown in FIG. 4. During such leftwardmovement of the piston 13, the first trigger port 34 passes back overthe outer porting tube seal 37 into communication with the supplypressure groove 29 prior to the second trigger port 35, but pressureflow to the right end of the valve bore 21 through the first triggerport 34 is initially blocked by the valve plunger. However, when thepiston 13 reaches its extreme left position shown in FIG. 4, the secondtrigger port 35 is pressurized by communication with the pressure groove29 thus pressurizing the right end of the valve bore 21 which forces thevalve plunger 20 to the left as shown in FIG. 5. When this occurs,pressure is again supplied to the head end 18 of the cylinder 12 throughthe valve plunger groove 36 and aligned piston ports 37 to cause thepiston 13 to move back to the right as shown in FIG. 2, and so on. Thestroke of the piston 13 is determined by the spacing between the triggerports 34 and 35 where they communicate with the porting tube bore 23,and such trigger ports are desirably located so that the piston does notbottom during movement in either direction.

The relative spacing between the trigger ports 34 and 35 in the valvebore 21 and spaced apart fluid seals 41 and 42 on the valve plunger 20which isolate the trigger ports from each other is also important toprevent pressure lock of the valve plunger and stall-out of the pumpunder all operating conditions. As best seen in F IG. 6, the triggerports 34 and 35 in the valve bore 21 are dimensionally positionedsomewhat further apart then the valve plunger seals 41 and 42 to allow asmall amount of flow to pass through the trigger ports when the valveplunger 20 is in the intermediate position shown. At that time, the headend 18 of the cylinder 12 is open to the vent passage 33 at the left endof the valve bore 21, allowing the piston 13 to be moved to the left bythe external applied force. Accordingly, even though the valve plunger20 may be in a pressurebalanced position preventing movement of thevalve plunger 20 during initial start-up of the pump, when the piston 13reaches the end of its stroke, both trigger ports 34 and 35 will bepressurized, whereby pressure will leak past both valve plunger seals 41and 42 and pressurize the right end of the valve bore 21, causing thevalve plunger 20 to move to the left. At that time normal operation ofthe pump will be resumed.

In FIG. 7 there is shown a modified form of pump 2' which issubstantially identical to the pump 2 of the FIGS. 2 through 6embodiment and accordingly the same reference numerals followed by aprime symbol are used to designate like parts. The only significantdifference is that the valve plunger 20' has been reversed and portingmodified to permit direct venting of the right end of the valve bore 21'to the continuously vented rod end 17' of the cylinder 12'. Otherwise,the details of construction and operation are substantially the same.

By using the small diameter porting tube 22 extending into the center ofthe piston 13 as previously described, the frictional seal forcesdeveloped during pump operation are greatly reduced. Moreover, usingresilient seals on all sealing lands rather than lap fits reduces costand improves efficiency by virtually eliminating leakage between movingparts. To insure adequate seal life, relatively small holes withcontrolled corner breaks may be used for the ports 34, 35 and 37 whichcross over seals and tefion capstrips may be used over the seals toresist abrasion.

However, it will be apparent that lap fits may be used between movingparts if desired. Moreover, it will also be apparent that the fixedporting tube despite its advantages may be eliminated and annulargrooves 43 and 44 provided in the inner wall of the main cylinder 12',one of which is pressurized and the other vented as shown in FIG. 8 toalternately pressurize and vent the head end 18' of the cylinder 12during shutting of the valve plunger 20' as before, Otherwise, thedetails of construction and operation of the pump of the FIG. 8embodiment are substantially identical to the pump 2 previouslydescribed, and the same reference numerals followed by a prime symbolare used to designate like parts. Of course, when the gas is ported tothe valve plunger 20' through the main piston 13 diameter as shown inFIG. 8, the seal frictional forces are much greater unless the parts arelap-fitted at additional expense.

In FIGS. 9 through 11 there is shown still another form of pump 45 inaccordance with this invention which is ported through the main piston46 diameter as in the FIG. 8 embodiment rather than through a centralfixed porting tube, and in addition, a spring 47 is utilized for returnmovement of the main porting valve plunger 48 to eliminate having to usea differential area plunger. The return spring 47 is interposed betweenthe left end of the main vale plunger 48 and a plug 49 in the left endof the main valve bore 50.

When the main piston 46 is in its extreme right position shown in FIG.9, the right end of the main valve bore 50 is vented through asupplemental valve plunger 51 contained in a supplemental valve bore 52in the piston 46. The right ends of both valve bores 50 and 52 areinterconnected by a passage 53 in the main piston 46, and thesupplemental valve plunger 51 has longitudinal and radial passages 54and 55 therein which provide communication between the right end of thesupplemental valve bore 52 and the vent'port 56 through a furtherpassage 57 in the main piston 46 and vent groove 58 when thesupplemental valve plunger 51 is in its extreme right position where itis biased by a spring 59 interposed between the left end of thesupplemental valve plunger 51 and a plug 60 in the left end of suchbore. The spring 47 is lighter than the spring 59, for a purpose to besubsequently described.

With the right end of the main valve bore 50 vented, the main valveplunger 48 is biased to its extreme right position shown in FIG. 9 bythe return spring '47, blocking both of the trigger ports 61 and62 andpermitting venting of the head end 63 of the main cylinder 64 through afirst passage 65 in the main piston 46 communicating the head end of thecylinder with the main valve bore and a second passage 66 in the piston46 communicating the main valve bore 50 with the rod end 67 of thecylinder which is continuously vented. When the pressure in the head endof the cylinder reaches a low enough level, the piston 46 will be causedto move to the left by the external applied force of the hydraulicreturn pressure or return spring 68 acting on the outer end of thepiston rod 69 as before.

FIG. 10 shows the piston 46 at its extreme left posi- '.tion at whichtime supply pressure atthe inlet 70 is ported to the right end of bothvalve plunger bores 50 and 52 by way of the pressure groove 71 andcommunicating passages 57, 53, 54 and in the piston 46 and supplementalvalve plunger 51. Both'ports 61 and 62 were opened to supply pressureprior to the passage 57, but flow therethrough was blocked by the mainvalve plunger 48.

With supply pressure at the right end of the main valve bore 50, themain valve plunger 48 is caused to move to the extreme left positionshown in FIG. 11, at which time port 61 is connected to the head end 63of the cylinder 64 through an annular groove 72 in the main valveplunger 48 and passage running between the head end of the piston andthe main valve bore 50. Venting of the head end of the cylinder 64through the passage '66 at that time is prevented by the main valveplunger 48 which blocks communication between the passages 66 and 65.

The supplemental valve plunger 51 is also urged to the left by thepressure in the right end of its associated bore 52, but not before themain valve plunger 48 has reached its extreme left position :due to thelighter spring 47 has reached its extreme left'position due to thelighter spring 47 acting thereon so that supply pressure is continued tobe supplied to the right ends of both valve bores 50 and 52 through thetrigger port 62 prior to the trigger port'57 being blocked by movementof the supplemental valve plunger 51. Pressure buildup in the head end63 of the cylinder 64 will cause the piston'46 to move once again to theright to complete the cycle, and the cycle will be repeated until thesupply pressure is interrupted.

From the foregoing, it will now be seen that each of the pump designsdisclosed herein includes a simplified valve and porting arrangementwhich permits efficient and reliable operation of the pump withoutchance of pump stall out. The valving for the pump is desirablycompletely contained within the piston for maximum simplicity and mostdirect porting of the head end of the cylinder, and the rod end of thecylinder is desirably always vented. A lowe external force continuouslyapplied to the piston rod causes return movement of the piston duringventing of the head end of the cylinder, and a simplified porting tubemay be utilized to supply pressure to the valve which greatly reducesseal friction.

I, therefore, particularly point out and distinctly claim as myinvention:

1. A reciprocating pump comprising a housing containing a cylinderincluding a rod end and a head end, a piston reciprocable within saidcylinder, a rod extending from said piston through the rod end of saidcylinder, valve means for supplying pressure to the head end of saidcylinder when said piston is adjacent said head end to cause movement ofsaid piston toward the rod end and for venting said head end when saidpiston is adjacent the rod end of said cylinder, and means for returningsaid piston toward said head end of said cylinder when said head end isvented as aforesaid, said valve means being contained in a valve bore insaid piston, and a porting tube extending from the head end of saidcylinder into an opening in said piston, said porting tube and pistonhaving communicating passages therein for supplying pressure throughsaid porting tube to said valve bore.

2. The pump of claim 1 further comprising vent means for continuouslyventing said rod end of said cylinder.

3. The pump of claim 1 wherein said means for returning said pistontoward said head end of said cylinder comprises a spring acting on theouter end of said piston rod.

4. The pump of claim 1 wherein said means for returning said pistontoward said head end of said cylinder comprises the hydraulic fluid tobe pumped which is maintained under low pressure.

5. The pump of claim 1 wherein said porting tube has a pair oflongitudinally spaced external seals in sealing engagement with the wallof said opening in said piston, and an external groove between saidseals, said porting tube also having a passage therein extending fromthe head end of said cylinder to said groove for supplying pressure tosaid groove, said piston having a first port between said valve bore andopening which provides communication between said groove in said portingtube and said valve bore, a vent passage communicating with said valvebore, and a second port extending from the head end of said piston tosaid valve bore, said valve means when disposed at one end of aid valvebore providing communication between said second port and said ventpassage for venting the head end of said cylinder, and when positionedat the other end of said valve bore establishing communication betweensaid first and second ports for supplying pressure to said head end ofsaid cylinder, and means for moving said valve mans from one end of saidvalve bore to the other when said piston reaches the ends of its stroke.

6. The pump of claim 5 wherein said means for moving said valve means asaforesaid comprises a pair of longitudinally spaced trigger ports insaid piston connecting said valve bore with said opening for saidporting tube said trigger ports being positioned to supply pressure fromsaid external groove in said porting tube to one end of said valve borewhen said piston is at the rod end of its stroke for moving said valvemeans to the other end of said valve bore, and also positioned to ventsaid one end of said valve bore to the inner end of said opening in saidpiston when said piston is at the head end of its stroke, a passage insaid piston venting the inner end of said opening to the rod end of saidcylinder, and means for moving said valve means to said one end of saidvalve bore upon venting said one end.

7-.- The pump of claim 6 wherein said means for moving said valve meansto said one end of said valve bore comprises a difierential area on saidvalve means less than the area of said one end of said valve means, andporting in said piston for supplying pressure to said valve bore whichacts on said differential area.

8. The pump of claim 6 wherein said means for moving said valve means tosaid one end of said valve bore comprises a spring interposed betweensaid valve means and the other end of said valve bore.

9. A reciprocating pump comprising a housing containing a cylinderincluding a rod end and a head end, a piston reciprocable within saidcylinder, a rod extending from said piston through the rod end of saidcylinder, valve means for supplying pressure to the head end of saidcylinder when said piston is adjacent said head end to cause movement ofsaid piston toward the rod end and for venting said head end when saidpiston is adjacent the rod end of said cylinder, and means for returningsaid piston toward said head end of said cylinder when said head end isvented as aforesaid, said valve means being contained in a valve bore insaid piston, and a pair of longitudinally spaced trigger ports in saidpiston which alternately vent and pressurize one end of said valve boreat opposite ends of the stroke of said piston to cause movement of saidvalve means for alternately venting and pressurizing the head end ofsaid cylinder.

10. The pump of claim 9 wherein said valve means has a pair oflongitudinally spaced seals which isolate said trigger ports from eachother for all positions of said valve means, said valve means when in apressure balanced position with respect to said trigger ports causingventing of the head end of said cylinder to permit movement of saidpiston toward the head end of said cylinder by the external appliedforce, the relative spacing between said trigger ports in said valvebore being somewhat greater than the relative spacing between said sealsin said valve means, whereby when said piston reaches the head end ofsaid cylinder, pressure is supplied to both of said trigger ports and ispermitted to leak past both of said seals to said one end of said valvebore, causing movement of said valve means toward the other end of saidvalve bore.

11. The pump of claim 9 wherein said valve means is caused to move tothe other end of said valve bore in response to pressure applied to saidone end of said valve bore, and means are provided for moving said valvemeans to said one end of said valve bore upon venting said one end.

12. The pump of claim 11 wherein said means for moving said valve meansto said one end of said valve bore comprises a differential area on saidvalve means less than the area of said one end of said valve means, andporting in said piston for supplying pressure to said differential area.

13. The pump of claim 11 wherein said means for moving said valve meansto said one end of said valve bore comprises a spring interposed betweensaid valve means and the other end of said valve bore.

14. A reciprocating pump comprising a housing containing a cylinderincluding a rod end and a head end, a piston reciprocable within saidcylinder, a rod extending from said piston through the rod end of saidcylinder, valve means for supplying pressure to the head end of saidcylinder when said piston is adjacent said head end to cause movement ofsaid piston toward the rod end and for venting said head end when saidpiston isadjacent the rod end of said cylinder, and means for returningsaid piston toward said head end of said cylinder when said head end isvented as aforesaid, said piston containing a first valve bore forreceipt of said valve means, and a second valve bore for receipt of asupplemental valve means through which one end of said valve means isalternately pressurized and vented when said piston reaches the ends ofits stroke, the pressurizing of said one end of said first valve borecausing said valve means to move to the other end of said first valvebore for pressurizing the head end of said cylinder, and means formoving said valve means to said one end of said first valve bore forventing the head end of said cylinder upon venting said one end of saidfirst valve bore.

15. The pump of claim 14 wherein said last-mentioned means comprises aspring interposed between said valve means and the other end of saidfirst valve bore.

16. The pump of claim 15 wherein said supplemental valve means ismovable by the pressure supplied to said one end of said first valvebore to a position blocking further pressure flow therethrough, and afurther spring stronger than the spring acting on said valve meansresists such movement of said supplemental valve means until said valvemeans has moved to a position permitting pressure to be supplied to saidone end of said first valve bore through a trigger port in said piston.

1. A reciprocating pump comprising a housing containing a cylinderincluding a rod end and a head end, a piston reciprocable within saidcylinder, a rod extending from said piston through the rod end of saidcylinder, valve means for supplying pressure to the head end of saidcylinder when said piston is adjacent said head end to cause movement ofsaid piston toward the rod end and for venting said head end when saidpiston is adjacent the rod end of said cylinder, and means for returningsaid piston toward said head end of said cylinder when said head end isvented as aforesaid, said valve means being contained in a valve bore insaid piston, and a porting tube extending from the head end of saidcylinder into an opening in said piston, said porting tube and pistonhaving communicating passages therein for supplying pressure throughsaid porting tube to said valve bore.
 2. The pump of claim 1 furthercomprising vent means for continuously venting said rod end of saidcylinder.
 3. The pump of claim 1 wherein said means for returning saidpiston toward said head end of said cylinder comprises a spring actingon the outer end of said piston rod.
 4. The pump of claim 1 wherein saidmeans for returning said piston toward said head end of said cylindercomprises the hydraulic fluid to be pumped which is maintained under lowpressure.
 5. The pump of claim 1 wherein said porting tube has a pair oflongitudinally spaced external seals in sealing engagement with the wallof said opening in said piston, and an external groove between saidseals, said porting tube also having a passage therein extending fromthe head end of said cylinder to said groove for supplying pressure tosaid groove, said piston having a first port between said valve bore andopening which provides communication between said groove in said portingtube and said valve bore, a vent passage communicating with said valvebore, and a second port extending from the head end of said piston tosaid valve bore, said valve means when disposed at one end of aid valvebore providing communication between said second port and said ventpassage for venting the head end of said cylinder, and when positionedat the other end of said valve bore establishing communication betweensaid first and second ports for supplying pressure to said head end ofsaid cylinder, and means for moving said valve mans from one end of saidvalve bore to the other when said piston reaches the ends of its stroke.6. The pump of claim 5 wherein said means for moving said valve means asaforesaid comprises a pair of longitudinally spaced trigger ports insaid piston connecting said valve bore with said opening for saidporting tube said trigger ports being positioned to supply pressure fromsaid external groove in said porting tube to one end of said valve borewhen said piston is at the rod end of its stroke for moving said valvemeans to the other end of said valve bore, and also positioned to ventsaid one end of said valve bore to the inner end of said opening in saidpiston when said piston is at the head end of its stroke, a passage insaid piston venting The inner end of said opening to the rod end of saidcylinder, and means for moving said valve means to said one end of saidvalve bore upon venting said one end.
 7. The pump of claim 6 whereinsaid means for moving said valve means to said one end of said valvebore comprises a differential area on said valve means less than thearea of said one end of said valve means, and porting in said piston forsupplying pressure to said valve bore which acts on said differentialarea.
 8. The pump of claim 6 wherein said means for moving said valvemeans to said one end of said valve bore comprises a spring interposedbetween said valve means and the other end of said valve bore.
 9. Areciprocating pump comprising a housing containing a cylinder includinga rod end and a head end, a piston reciprocable within said cylinder, arod extending from said piston through the rod end of said cylinder,valve means for supplying pressure to the head end of said cylinder whensaid piston is adjacent said head end to cause movement of said pistontoward the rod end and for venting said head end when said piston isadjacent the rod end of said cylinder, and means for returning saidpiston toward said head end of said cylinder when said head end isvented as aforesaid, said valve means being contained in a valve bore insaid piston, and a pair of longitudinally spaced trigger ports in saidpiston which alternately vent and pressurize one end of said valve boreat opposite ends of the stroke of said piston to cause movement of saidvalve means for alternately venting and pressurizing the head end ofsaid cylinder.
 10. The pump of claim 9 wherein said valve means has apair of longitudinally spaced seals which isolate said trigger portsfrom each other for all positions of said valve means, said valve meanswhen in a pressure balanced position with respect to said trigger portscausing venting of the head end of said cylinder to permit movement ofsaid piston toward the head end of said cylinder by the external appliedforce, the relative spacing between said trigger ports in said valvebore being somewhat greater than the relative spacing between said sealsin said valve means, whereby when said piston reaches the head end ofsaid cylinder, pressure is supplied to both of said trigger ports and ispermitted to leak past both of said seals to said one end of said valvebore, causing movement of said valve means toward the other end of saidvalve bore.
 11. The pump of claim 9 wherein said valve means is causedto move to the other end of said valve bore in response to pressureapplied to said one end of said valve bore, and means are provided formoving said valve means to said one end of said valve bore upon ventingsaid one end.
 12. The pump of claim 11 wherein said means for movingsaid valve means to said one end of said valve bore comprises adifferential area on said valve means less than the area of said one endof said valve means, and porting in said piston for supplying pressureto said differential area.
 13. The pump of claim 11 wherein said meansfor moving said valve means to said one end of said valve bore comprisesa spring interposed between said valve means and the other end of saidvalve bore.
 14. A reciprocating pump comprising a housing containing acylinder including a rod end and a head end, a piston reciprocablewithin said cylinder, a rod extending from said piston through the rodend of said cylinder, valve means for supplying pressure to the head endof said cylinder when said piston is adjacent said head end to causemovement of said piston toward the rod end and for venting said head endwhen said piston is adjacent the rod end of said cylinder, and means forreturning said piston toward said head end of said cylinder when saidhead end is vented as aforesaid, said piston containing a first valvebore for receipt of said valve means, and a second valve bore forreceipt of a supplemental valve means through which one end of saidvalve means is alternately prEssurized and vented when said pistonreaches the ends of its stroke, the pressurizing of said one end of saidfirst valve bore causing said valve means to move to the other end ofsaid first valve bore for pressurizing the head end of said cylinder,and means for moving said valve means to said one end of said firstvalve bore for venting the head end of said cylinder upon venting saidone end of said first valve bore.
 15. The pump of claim 14 wherein saidlast-mentioned means comprises a spring interposed between said valvemeans and the other end of said first valve bore.
 16. The pump of claim15 wherein said supplemental valve means is movable by the pressuresupplied to said one end of said first valve bore to a position blockingfurther pressure flow therethrough, and a further spring stronger thanthe spring acting on said valve means resists such movement of saidsupplemental valve means until said valve means has moved to a positionpermitting pressure to be supplied to said one end of said first valvebore through a trigger port in said piston.